For majority of the partition chassis in 2-socket enterprise partition platforms (EPPs), processor cores are allocated one by one. (See for example Visual Aid for Understanding Chassis and Cores They Can Include.)
But the 2-socket, 20-core EPP has so many processor cores that there are more cores than partitions and partition chassis. Therefore, processor cores for Chassis-A are in some cases allocated two at a time (that is, in a 2-core granularity).
Chassis Configurations for a 2-Socket, 20-Core EPP shows the chassis configuration for a 2-socket, 20-core EPP. The following fragment shows how such chassis and cores are represented. This fragment shows that
Core 26 is allocated as a single unit of one.
Cores 27 to 28 are allocated as a single unit of two.